And Harper says he doesn't mind standing alone in the stridency of his denunciations of the Russian president.
Still, in an interview with Global News' The West Block, Harper backed off comparing Russia's invasion of Ukraine's Crimea region to Nazi Germany's occupation of Sudetenland, a prelude to the Second World War.
"He's obviously a nationalist, an extreme nationalist, and he's obviously an imperialist," Harper said in the interview, broadcast Sunday.
"This is an individual who clearly believes that, if he's able, he has the right and the ability to invade another country, to alter borders through military force.
"We're not at Hitleresque proportions," he added. "But this is really disconcerting. This is a major power threatening global peace and security in this way and I don't think it's to be taken lightly."
In France to mark the 70th anniversary of D-Day last week, Harper shunned Putin while a number of other G7 leaders, including U.S. President Barack Obama, met with the Russian president.
Harper noted that Putin was not invited to the G7 leaders' summit just prior to the D-Day commemoration. And he said all the leaders agreed that in meetings with the Russian president their message needs to be consistent: get out of Crimea.
Still, he said he's not afraid of taking a harder line against Putin than other leaders.
"We're not afraid to take stands that then put us offside others, from time to time."
Harper got some support for his hard line from Australian Prime Minister Tony Abbott, who arrived Sunday in Ottawa for his first official visit.
"They're very forthright remarks and I think that they're perfectly appropriate remarks for the Canadian prime minister to make," Abbott said of Harper's description of Putin.
While Australia has less involvement than Canada in the affairs of Eastern Europe, Abbott said: "Let's not minimize the affront to international stability and the affront to the ordinary norms of behaviour between nations that Russia has been responsible for."
"... This should stop. No country has a right to bully another country just because it can."
While he backed off previous comparisons of Putin to Adolph Hitler, Harper said the Second World War could have been avoided if other nations hadn't turned a blind eye to Germany's aggression — a lesson he believes applies to Russia today.
"Our foreign policy obviously incorporates those kinds of lessons."
Harper said his government is considering permanently stationing troops in Eastern Europe, as part of a possible NATO bid to reassure allies in the region.
"Obviously, our Eastern European allies are, to say they're alarmed by what's happened in Ukraine is an understatement. They are, quite frankly, beside themselves and obviously very, very worried," he said.
"So obviously we're dialoguing with the Poles and others about what we can do as NATO allies to provide reassurance on an ongoing basis."